Journal of the Minnesota Academy of Science

Volume 3 Number 3 Article 4

1891

The Lower Silurian Formations of Wisconsin and Minnesota Compared

F. W. Sardeson

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Recommended Citation Sardeson, F. W. (1891). The Lower Silurian Formations of Wisconsin and Minnesota Compared. Journal of the Minnesota Academy of Science, Vol. 3 No.3, 319-326. Retrieved from https://digitalcommons.morris.umn.edu/jmas/vol3/iss3/4

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The fossils are for the most part, marked out by discolora· tion (brown or red), but a few by cleavage only. They are quite numerous and are easily found when one once knows how and where to look for them. They occur fifty or more feet below the top of th~ formation. I have assigned the specimens found, to the genera anu species to which I think they belong. They are remarkably like species found in the lower part of the Trenton shales and in the Trenton limestone which here rests conformably on the Saint Peter sandstone. And it may be, as has been suggested, that, the Saint Peter is of the Silurian rather than that of the Cambrian formation. As soon as spring opens, I shall spend some days in a more thorough search, in order to find out as far as possible, the true nature and horizon of these fossils in the Saint Peter sandstone. February 3, 1891.

THE LOWER SILURIAN FORMATIONS OF WISCONSIN AND MINNESOT.\ COMPARED.-F. W. Sardeson. It is the purpose of this paper to give some observations on the Silurian of Minnesota, and the Trenton group in particular; and to compare it with the same of Wisconsin. There are some difficulties in undertaking such a comparison. For example, the Trenton group in Wisconsin is nearly all lime· stone, while in Minnesota it is largely composed of shales. This lithological difference is accompanied by some differences in the fauna and in the outward appearance of the fossils. Then, too, four beds are recognized in the Trenton of vVisconsin, the Lower Buff, Lower Blue, Upper Buff and Upper Blue beds, while in Minnesota two are usually spoken of-Trenton limestone, or shell beds, and Trenton shales, or green shales. These difficulties I shall aim to a'·oid in part and in part explain. I shall take up one by one the beds as seen in Minnesota and compare them with the same in vVisconsin, so far as I can. The lower Trenton limestone, or· Trenton limestone of :Min· nesota, consists of three beds differing somewhat in lithological character and fauna; most strongly so in the area around the "Twin Cities," i. c., Minneapolis and Saint Paul. The first of these, next to and conformable with the Saint Peter sandstone, is the same bed as the Lower Buff limestone of Wisconsin, judg-

Digitized by Coogle 320 Lo<;.•a Silurian of Wisconsin a11d .lliuucsota. ing from its fauna, a variety of Orthis suboeqttata Con., 0. de­ flecta Con., Rhynchonel/a orienta/is Bill., Strophomena mimuso­ tensis Winchell (varieties), Ambonychia attmuata Han, etc. At Janesville, Wis., this bed is about eighteen feet in thick­ ness, in Jefferson county, Wis., about fourteen feet; in LaFayette Co., ten feet; at Dodgeville and Platteville, Wis., nearly twent\' feet. In Fillmore Co., Miimesota, it is about twelve feet; at Rochester, Olmsted Co., the same; at Minneapolis, fifteen feet; at Faribault, Rice Co., it is either represented by four and one­ half feet of green (apparently unfossiliferous) shale, or it is absent. The .second of the three limestone beds at :\linneapolis (seven feet of carbonaceous limestone) preserves few fossil3 well. In nearly an other respects it is like the bed above rather than the one below it. But I am not so sure that this is the case elsewhere. At Faribault the seven feet immediately on the green shale mentioned above has characteristics of this bed. In south· eastern Minnesota it is less easily distinguishable from the bed be­ low. In southwestern Wisconsin I could not find it at all, unless it is there more fossiliferous, and hence confused with the strata above. But at other points in Wisconsin, these strata seem to be distinguishable though in every case less distinctly than at Min­ neapolis. The five feet three inches at the top of the limestone at l\Iin­ neapolis is the fossiliferous bed of the three. The species that occur in it are essentially the same fonns as those common in the Lower Blue bed of \Visconsin. Orthis pervcta Con., Trochonema beloitellse \Vhitf., Cypricardites rectirostris H., are abundant fonns. At Faribault this bed is of about the usual depth for Minne­ sota, but is darker and more carbonaceous than usual and brachio­ pod shells are well preserved. In Wisconsin, at Platteville, ~Iin­ eral Point and Dodg-eville, it seemed to be from ten to fifteen feet thick and perhaps the same thickness at Janesville. The rest of the Lower Trenton in Minnesota is !'hale which is supposed to· be mainly the equivalent of the Cpper Buff an

Digitized by Coogle Lo'i;.'cr Silurian of Wisconsin and .llfiunesota. 32I nesota are the equivalents of the Upper Buff and Blue beds in Wisconsin, with the exception of the first ten feet of our shales. These last are, doubtless, a part of the Lower Blue bed in Wis­ consin. I came to the conclusion, some time ago, that the first strata of the shales could be classed with the limestone below, as easily as with the shales above. They form here a transitional bed, which for convenience I wish to distinguish as the Stictoporel/a bed. The limestone strata, which constitutes part of it, though crystalline like the slabs in the true shales above, are the result of sedimentation like the limestones below. The fauna, too, is as much that of the preceding as of the succeeding strata. The Stictoporclla bed, in ascending order, is as follows: Limestone 6 in., limestone I ft. I in., limestone 2 ft., shale I ft. 6 in., limestone 7 in., shale 5 ft., lime!'tone I ft. 6 in. (measure­ ments taken at Saint Paul.) The shaly parts are not unmixed day, but have numerous thin hard calcareous laminre in them. The stone and shale vary locally in thickness and alternation, but are of about the same proportion, as seen in Goodhue, Olmsted and Fillmore counties, Minnesota. There is a bed of dark colored limestone upon the Lower Blue bed at Platteville, Wis., which appears to be the same as the Stictoporella bed in Minnesota. It consists of, first, about four feet of solid strata, with thinner clea,·ablc strata of the same col­ or; second, three to four feet of green shale such as commonly occurs in ~dinnesota; and third, four feet of dark colored stone, apparently the transitional back to the ordinary limestone. The fossils were most of them characteristic forms of the Stictoporella bed in Minnesota. At Dodge,·ille, Wis., the same strata, so far as I could judge, occur: as a light brown bed about ten feet thick, but quite unfos­ siliferous. At the time this place was examined, I was very much puzzled as to whether this bed belong-ed to the Lower Blue or l'pper Buff limestone, but upon reading over the Geology uf \Vis­ consin, Vol. I, I became quite satisfied that it would be classified as equivalent to part of the Lower Blue limestone of the Rock river valley. Along the Illinois Central railroad lll'ar Dodgeville, nearly every stratunf from the Saint Peter to the middle of the Galena is clearly exposed. There can be r{'('og-nized the Lower Buff,

Digitized by Coogle 322 Lower Silurian of Wisconsin and Mim1csota.

Lower Blue including the bed mentioned above, the Upper Buff and Upper Blue beds. But neither there nor in the Rock river valley do any strong evidences appear to prove that the Upper Buff and .Upper Blue beds are equivalent to the Trenton shales of Minnesota. Only their position suggests that they are very probably equivalents. The Trenton shales are about eighty feet thick at Saint Paul. In Goodhue county they are about the same. In Fillmore county they are much thinner mainly on account of the upper strata, as exposed in Saint Paul, being here represented by fifteen or twenty feet of limestone. So far as known the beds of the shales were co-extensive in deposition in Minnesota. The first ten feet of Trenton shales has been described above as the Stictoporella bed. It is very fossiliferous; Orthis sub­ aequata var., gibbosa Bill., Anoloteichia impolita Ulr., Pachydictya foliata Ulr., and Stictoporella frondifcra Ulr., occur in masses. The last named is unknow~ except in this bed and is very widely distributed; and for that reason the name Stictoporella has been proposed for this ten feet of shale. The next thirty feet is of unifom1 dark green unctuous shale with numerous fossils, but many of them poor on account of the nature of the matrix. But here and there, especially toward the top of the bed, are reefs of bryozoa and brachiopoda, as well as scattering individuals of mollusca, molluscoidea and crelenterat:t throughout. These reefs occur as crystalline slabs from one to four inches thick and of various extent; from their appearance they are easily taken for sedimentary strata. But they have probably been formed by the infiltration of calcium carbonate into lenticular beds of fossils, as shown by the irregular cementing together of the fossils and more particularly. by being made up almost en­ tirely of animal remains. Also where a large shell lies horizon­ tally near the under surface of a slab there will usually be a shaly spot or core under the shell as if it had shed off the infiltration from above. Mollusca are preserved as blue calcareous casts. I could scarcely characterize this bed by enumerating the species which occur in it. An undescribed rhynchonella, which is essentially R. increbescens Hall without the concentric lines, oc­ curs from the top of this bed downwards. R. ainsliei Winchell has a like distribution, but is more local. Only thr'ee species of orthis, 0. sttbacquata Con., 0. testudinaria Dalman (variety) and

o;9,tized by Coogle Lo<.;.•cr Silurian of W isconsi" atJd M inncsota. 323

Q. tricc11aria Con., are known to occur, Stictopora mutabilis Ulr., though perhaps not confined to these strata, yet occurs in such numbers here that the name Stictopora seemed appropriate in designating this bed. The next twenty feet is the Fucoid bed. The shale is made up largely of fucoidal masses and of calcareous laminre some­ what irregularly distributed. At Saint Paul there is, first, I 5 feet of shale; then 5 in. lime­ stone; 18 in. shale ; 3 ft. of hard crystalline strata that weather reddish brown. This bed will be recognized at once in Ramsey, Dakota and Goodhue counties, by the quantity of fucoidal re­ mains or by the occurrence of Phylloporina corticosa Ulr. In Fillmore county I have found the evidences of this bed but never the bed itself exposed. Orthis pectitlella Con. occurs for the first time at this horizon and also Rhyncho1ulla increbescens Hall. The strata that lie upon the Fucoid bed are without fucoidal remains but are much more fossiliferous upon the whole. Z ygos­ pira recurvirostris Hall, Rhy11chonc/la increbescens Hall, are ex­ tremely abundant. This bed is characterized by the absence of certain species that occur above or be!O\v. The name Zygospira bed is proposed from the most numerous species. There is in this bed three feet of hard shale, four feet of ordinary shale, and one foot of shaly limestone-eight feet in all. At the upper limit of the Zygospira bed a new fauna appears. Receptaculites, Platystrophia, Orthisina-which are unknown be­ low. Orthisi11a [ H emipro11ites] americana Whit£., has not been found except in this bed and hence the name Orthisina is pro­ posed. Lithologically the Zygospira and Orthisina beds are difficult to distinguish, and later searching may succeed in merging the former into the latter. In both, there are peculiar calcareous lumps, irregular in shape, rounded in the latter bed and of lighter color than the other parts of the shale, and alternating in strata, with thin smooth surfaced limestone layers. At Saint Paul only part of the Orthisina bed remains. At Kenyon, Goodhue county, there is over fifteen feet of this bed exposed. In Olmsted and Fill­ more counties the exposures of this bed are rare, and these do not show more than ten feet thickness. In the last named county the bed consists rather of shaly limestone than a shale. Just where the division between the Trenton and Galena should be drawn ·so as to agree with the division in Wisconsin is

Digitized by Coogle 324 Lower Silurian of Wisconsin and M i1111csota. not deterlnined. I saw the lowest strata of the Galena at Evans­ ville, Dodgeville, etc., in Wisconsin, and formed the opinion that the corresponding line in Minnesota, should be drawn at the top of the Orthisina bed: but the fauna here seems to demand a divis­ ion at the top of the Zygospira bed or below it. From the top of the Orthisina bed to the upper limit of the Trenton group is fully 100 ft. in Minnesota, and in many places the rock is exposed in vertical walls, for part or all its thickness. The fauna in the Galena formation is much more meager than in Trenton, and for that reason close division into beds is less easily made and with less certainty. Three subdivisions however have been noted in Minnesota. 1. The first of these is a somewhat carbonaceous limestone, about thirty feet thick, that crumbles more or less in weathering. The name Camarc/la bed seems appropriate on account of several species of that genus, which occur here and have not been found in other beds. 2. The next twenty feet is of a finn, very durable limestone with few fossils of several species. Inarticulate brachiopods arc well preserved. From the occurrence here of Lingulasma sclw­ cherti(!) Ulr., the name Litlgulasma bed has suggested itself. 3· The last fifty feet of the Galena formation, which I shall ::all the Mac/urea bed, is characterized by large gastropoda­ Murchisonia major Hall, Fttsispira elongata Hall, M aclurea cuneata Whit£., Raphistoma lmticularis (large variety) . This Maclurea bed I feel safe in correlating with the last de­ posited strata of the Galena formation in Wisconsin, both on lith­ ologic and palreontologic grounds. And indeed no reason is known to me for supposing that the strata composing the Trenton group in Minnesota and Wisconsin were not in every case con­ tinuously and contemporaneously deposited. As to the first and the last bed, identical fossil forms and similarity in structure, to­ gether with nearly continuous exposures, furnish sufficient evi­ dence of their relation on both sides of the Mississippi valley. The succession of strata through the Trenton group in Min­ nesota forms a gradual transition from limestone to typical shale and again back to limestone. In \Visconsin this transition is less and less marked in proportion to the distance from the northwest­ ern extension of the strata in Minnesota, i. e. from the line of the advancing and retreating shore of the sea in which the Trenton sediments were deposited.

Digitized by Coogle Lower Silurian Fauna of Minnesota .

. .ll6lfOIIall

FIGURE 8.

Figure 8 is a diagrammatic section of the Lower Silurian (Ordovician) rocks of south­ eastern Minnesota, showing the position of the beds just described, their thickness and their lithologic charac­ ters so far as practicable. The Saint Peter is includ­ ~ ed in the Lower Silurian but ~ is partly excluded from the f--JL,--L.....-"o-.L-.1 ~ diagram for lack of space. Various combinations of the words Trenton etc., (on the extreme left) and lime­ stone etc., (on the right of the diagram) have been and are still in use in Minnesota. But such names are based mainly on lithologic charac­ ter and conjecture and are scarcely more than provi­ sional. The division between GALENA and TRENTON is in­ definite. Trenton group and Galena group are synonyms. .. So also are Cincinnati group ~ and Hudson River group. ~..., .~ ~.,_,..,....,,...,...~~ ~ o:; Salnt ht•r J6o'

Digitized by Coogle • 326 Lower Silurian Fauna of Minnesota.

In Fillmore county the Galena is surmounted by twenty, or more, feet of shales, the Maquoketa. These shales in Minnesota consist of alternating strata of shale and crystalline limestone, and are continuous with the Maquoketa of Iowa. But the fossils consist of numerous specimens of a few species of brachiopoda and bryozoa almost wholly different species from what I was able to find in the Maquoketa near Dubuque, Iowa; yet they are not forms that indicate a difference in the age of the strata exposed near Dubuque, and those exposed in Fillmore county. • Lastly there is upon the Maquoketa shales from fifty to seventy feet of limestone that is very fossiliferous-0. subquad­ rata Hall, 0. testudinaria (three varieties, 0 . whitfieldi Winchell (0. kankakensis McChesney,) Rhynchonella capax Con., Strep­ torhynchus wisconsensis Whit£., etc. The fauna appears to be­ long to the Cincinnati group. But this limestone appears to be ~ontinuous with that identified in Iowa as belonging to the Niag­ ara group, though perhaps only in part. In order to avoid confu­ sion in subsequent work, I would propose a new name for this limestone-Wykoff beds-from the town near which the best ex- posure known occurs. . I have never seen any exposures nor any fossils of the Upper Silurian in Minnesota. The Devonian lies unconformably upo!l the Wykoff beds. Indeed if the Devonian limestone, as is prob­ able, extended much further north than it now does, and in the same manner, it rested unconformably upon the Galena or even the Lower Trenton formations within thirty to fifty miles of its present northern limit. As it now lies there it is much less than fifty feet between the top of the Maquoketa shales, and the base of the Devoni.an at Spring Valley, while only fifteen miles further south near the boundary of Iowa ·and Minnesota, at least seventy-five feet of limestone intervenes. October 6, 1891.

THE ~ANGE AND DISTRIBUTION OF THE LOWER SILURIAN FAUNA OF MINNESOTA WITH DESCRIPTIONS OF SOME NEW SPECIES.- F. W Sardeson. My first intention was to make out a list of Palreozoic fossils found in Minnesota, with notes on their distribution and vertical range as revealed by thirty or forty exposures. But during prep-

Digitized by Coogle